The millimeter-wave portion of the radio spectrum, e.g., including frequencies in the range of 60 GHz or so, is expected to provide many benefits for fifth-generation (5G) wireless communications systems. The accessible channel bandwidths in the millimeter-wave band are potentially larger than those available to commercial wireless systems operating at lower bands, such as those systems using the current Long-Term Evolution (LTE) radio access technologies developed by the membership of the 3rd-Generation Partnership Project (3GPP). Further, the smaller radio wavelengths in the millimeter-wave band allow the radio transceivers to have more compact hardware when supporting multiple antennas, due to the fact that the antenna element separation in a multi-antenna scenario is typically proportional to the wavelength.
One implication of this latter advantage of the millimeter-wave band is that a large number of antenna elements can be connected to radio access nodes (hereinafter referred to as simply access nodes, or ANs) while maintaining relatively “regular” sizes, compared to previously deployed nodes. These antenna elements, which may be arranged as arrays of horizontally and/or vertically spaced elements, may be used to form narrow beams, concentrating, for example, all the access node's transmit power in a specific/desired direction. This way, distant wireless devices (commonly called user equipments or UEs in 3GPP specification documents) can be reached without causing high interference to devices other than those specifically targeted by the access nodes.
On the other hand, initial synchronization of UEs in a millimeter-wave wireless network, which is generally the first step taken by UEs when attempting to access the network for services, raises some challenging issues. In this context, a key challenge is ensuring that whenever a new UE tries to join such a network, at least one AN in the vicinity of the UE should be able to provide it with signal quality that is good enough to establish a connection between the AN and the UE. At the same time, however, as little power as possible should be devoted to signals transmitted to the UE, so as to allow for support of many other UEs, to minimize interference to other devices, and to minimize overall power consumption of the network. However, the use of narrow beams for initial synchronization purposes provides good signal quality only to a small fraction of the area to be covered. If a new UE arrives in a poorly-covered area, one consequence of using narrow beams for the transmission of synchronization signals can be that this UE is unable to join the network, since it cannot listen to and decode satisfactorily any synchronization signal from the ANs.
Inactivating antenna elements in the array of antenna elements available to a given access node makes it possible to create wider beam patterns, with the limit being reached by transmission from only a single antenna element. In some possible configurations, however, such as configurations in which there is one power amplifier per antenna element, this may entail a reduction in total conducted power into the array. It is therefore more power-efficient, as a general matter, to use all of the available antenna elements to radiate power.
Assuming the use by each access node in a given region of all the antenna elements available to it, it is therefore desirable to minimize the total consumed power by adjusting the power level of every beam, from every access node, in each transmit-time interval, so that every UE in the region can perform initial synchronization. This leads to a beam-sweep procedure, in which narrow beams, one at each AN, are simultaneously transmitted in contiguous transmit-time intervals, namely beam sweep instances, in order to radiate energy over the area where UEs may appear and try to establish connection, until the whole area is scanned.
To minimize the total consumed power for this initial synchronization process, an optimization problem can be formulated where the objective is to minimize the sum of all the transmit power levels to be used during the beam scan. The objective is subject to a set of quality-of-service (QoS) constraints (e.g., minimum signal-to-interference-plus-noise ratio (SINR) requirements for the reception of synchronization signals by UEs in the region) and a set of transmit power constraints. The result is the feasible power settings, for all transmitted synchronization beams, where the element-wise sum of the power settings is minimized. From the resulting power settings, the beam sweep pattern to be used in the beam scan is derived. Ideally, the transmission of synchronization signals using the resulting power settings provides good signal quality to any new UE in the area that may try to synchronize to an AN.
By its nature, the problem described above is combinatorial, with high computational complexity for large-scale networks. Thus, its optimal solution is difficult to obtain, as the exhaustive search for the optimal solution grows exponentially with the number of QoS constraints. Although the combinatorial problem can be transformed into a mixed-integer linear program to be solved in a more efficient manner, it is still intractable for very-large-scale networks.
One possible low-complexity approach to defining beam-sweeping patterns for the ANS is to choose the beam sweep pattern, i.e., the sequence of beams that ANs follow over beam sweep instances, at random, with no power optimization. This is the approach described in C. N. Barati, S. A. Hosseini, S. Rangan, P. Liu, T. Korakis, and S. S. Panwar, “Directional cell search for millimeter wave cellular systems,” in IEEE 15th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), June 2014]. However, this approach would make ANs transmit synchronization signals to areas where UEs never (or rarely) appear. Thus, unnecessary power is consumed. In addition, UEs may experience high interference levels from the randomly swept beams.
Another approach is to design the beam pattern and optimize the power settings based on historical statistics for UEs, as collected by the wireless network. This is the approach described in I. M. Guerreiro, J. Axnäs, D. Hui, and C. C. Cavalcante, “Power-efficient beam sweeping for initial synchronization in mm-Wave wireless networks,” in IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), June 2015. From such a data set, ANs sub-optimally find a power setting sufficient to provide good synchronization signal quality for UEs to synchronize. As a consequence, the beam sweep pattern is derived from power settings, where only directions associated with non-zero power levels are used. However, the specific algorithms detailed in the paper cited above are performed in a centralized manner, and provide solutions far from the optimum.